。
She arranged the flowers beautifully.
Literal
She [topic-は] flowers [object-を] beautifully arranged.
きれいに is the adverbial form of the na-adjective きれい ('pretty, beautiful, clean'), and it modifies the verb's manner: 'arranged beautifully.' This is the standard way to turn a na-adjective into an adverb — drop な, add に. The verb 生ける is the same one used in 生け花 (ikebana), and saying 花をきれいに生ける evokes not casual flower-sticking but the considered, line-and-balance art of flower arrangement that traces back through the tea ceremony and Buddhist temple offerings to the medieval period.